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Publication numberUS5087956 A
Publication typeGrant
Application numberUS 07/358,261
Publication dateFeb 11, 1992
Filing dateMay 30, 1989
Priority dateOct 25, 1985
Fee statusPaid
Publication number07358261, 358261, US 5087956 A, US 5087956A, US-A-5087956, US5087956 A, US5087956A
InventorsShuji Ikeda, Satoshi Meguro, Kotaro Nishimura, Sho Yamamoto, Nobuyoshi Tanimura
Original AssigneeHitachi, Ltd.
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Semiconductor memory device
US 5087956 A
Abstract
An SRAM including a memory cell having a high-resistance load element. The load element is formed from a polysilicon film, and an impurity is introduced into at least a part of the polysilicon film for the purpose of increasing the threshold voltage of a parasitic MISFET formed using the load element as its channel region. Alternatively, the deposition of the polysilicon film is carried out at a relatively high temperature, thereby preventing any increase in the current flowing through the load element, and thus reducing the power dissipation in the SRAM.
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Claims(37)
What is claimed is:
1. A memory cell structure formed on a semiconductor substrate having a flip-flop comprised of first and second inverters, output terminals of said first and second inverters being respectively connected to input terminals of said second and first inverters, each of said first and second inverters being comprised of a MISFET and a polysilicon film which is connected in series to said MISFET, said polysilicon film being connected to a first wiring line for supplying a first fixed potential to said memory cells and having a plurality of first regions of a first conductivity type, a second region which has higher resistivity than that of said first regions, and a third region of a second conductivity type, wherein said second and third regions are disposed between said first regions.
2. A memory cell structure according to claim 1, wherein said third region is disposed closer to said MISFET than said second region.
3. A memory cell structure according to claim 2, wherein said polysilicon film further includes a fourth region of the second conductivity type, said fourth region being formed between said second region and one of said first regions which is disposed adjacent to said MISFET.
4. A memory cell structure according to claim 1, wherein said first and second conductivity types are respectively n- and p- types, said first region being doped with n-type impurity and said third region being doped with p-type impurity.
5. A memory cell structure according to claim 4, wherein the n-type impurity concentration of said second region is lower than that of said first region, and the p-type impurity concentration of said second region is lower than that of said third region.
6. A memory cell structure according to claim 5, wherein the n-type impurity concentration of said first region is higher than the p-type impurity concentration of said third region.
7. A memory cell structure according to claim 6, wherein said second region is an intrinsic region.
8. A memory cell structure according to claim 1, wherein a data line is stacked over at least a part of said second and third regions.
9. A memory cell structure according to claim 8, wherein a gate electrode of said MISFET is disposed below at least a part of said second and third regions.
10. A memory cell structure according to claim 8, wherein said data line is one of complementary data lines, and wherein a pair of said complementary data lines are coupled to said memory cell and are stacked over at least a part of said second and third regions of said first and second inverters respectively.
11. A memory cell structure according to claim 1, wherein said first wiring line is a power supply voltage wiring and is made integrally with said polysilicon film.
12. A semiconductor memory device comprising:
a memory cell, formed on a semiconductor substrate, including a flip-flop comprised of first and second inverters, output terminals of said first and second inverters being respectively connected to input terminals of said second and first inverters, each of said first and second inverters being comprised of a MISFET and a polysilicon film which is connected in series to said MISFET, said polysilicon film being connected to a first wiring line which serves to supply a power supply voltage to said memory cells and having a first region of a first conductivity type, a second region which has higher resistivity than that of said first region, and a third region of a second conductivity type; and
a pair of complementary data lines coupled to said memory cell and stacked over at least a part of said second and third regions of said first and second inverters respectively.
13. A semiconductor memory device according to claim 12, wherein said first region is disposed closer to said first wiring line than at least said second and third regions, and said third region is disposed closer to said MISFET than at least said second region.
14. A semiconductor memory device according to claim 13, wherein said polysilicon film further includes a fourth region of the first conductivity type, said fourth region being formed on a side of said polysilicon film which is closer to said MISFET than said third region, and having the same impurity concentration as that of said first region.
15. A semiconductor memory device according to claim 14, wherein said polysilicon film further includes a fifth region of the second conductivity type, said fifth region being formed between said first and second regions and having the same impurity concentration as that of said third region.
16. A semiconductor memory device according to claim 12, wherein said first and second conductivity types are respectively n- and p- types, said first region being doped with n-type impurity and said third region being doped with p-type impurity.
17. A semiconductor memory device according to claim 16, wherein the n-type impurity concentration of said second region is lower than that of said first region, and the p-type impurity concentration of said second region is lower than that of said third region.
18. A semiconductor memory device according to claim 17, wherein the n-type impurity concentration of said first region is higher than the p-type impurity concentration of said third region.
19. A semiconductor memory device according to claim 18, wherein said complementary data lines extend in substantially the same direction as that of said second and third region of said polycrystalline silicon films.
20. A semiconductor memory device according to claim 18, wherein said second region is an intrinsic region.
21. A semiconductor memory device according to claim 12, wherein a gate electrode of said MISFET is disposed below at least a part of said second and third regions.
22. A semiconductor memory device according to claim 12, wherein said first wiring line is made integrally with said polysilicon film.
23. A semiconductor device including a static RAM cell each having cross-coupled first and second driver MISFET, first and second switching MISFETs being connected between said first and second driver MISFETs and complementary data lines and having their gate electrodes connected to a word line, and polysilicon films which are connected in series to said first and second driver MISFETs, each of said polysilicon film being connected to a first wiring line which serves to supply a power supply voltage to said cells and including a first region of a first conductivity type and a load element region, said load element region having a second region which has a higher resistivity than that of said first region, wherein said load element region further includes at least a third region of a second conductivity type, and wherein complementary data lines are stacked over at least a part of said load element region of said polysilicon films.
24. A semiconductor device according to claim 23, wherein said first region is disposed closer to said first wiring line than at least said second and third regions, said third region being disposed closer to said first and second MISFETs than at least said second region.
25. A semiconductor device according to claim 24, wherein said polysilicon film further includes a fourth region of the first conductivity type, said fourth region being formed on the side of said polysilicon film which is closer to said first and second MISFETs than said third region, and having the same impurity concentration as that of said first region.
26. A semiconductor device according to claim 25, wherein said polysilicon film further includes a fifth region of the second conductivity type, said fifth region being formed between said first and second regions and having the same impurity concentration as that of said third region.
27. A semiconductor device according to claim 23, wherein said first and second conductivity types are respectively n- and p- types, said first region being doped with n-type impurity and said third region being doped with p-type impurity.
28. A semiconductor device according to claim 27, wherein the n-type impurity concentration of said second region is lower than that of said first region, and the p-type impurity concentration of said second region is lower than that of said third region.
29. A semiconductor device according to claim 27, wherein the n-type impurity concentration of said first region is higher than the p-type impurity concentration of said third region.
30. A semiconductor device according to claim 29, wherein said complementary data lines extend in substantially the same direction as that of said load element regions.
31. A semiconductor device according to claim 29, wherein said second region is an intrinsic region.
32. A semiconductor device according to claim 31, wherein said third region has a predetermined p-type impurity concentration so as to enhance the threshold voltage of a parasitic MISFET having a channel region defined by said load element region.
33. A semiconductor device according to claim 31, wherein each of said complementary data lines is formed above said semiconductor substrate and is positioned so as to be substantially aligned over said load element regions of said polysilicon films such that a projection of each data line in the direction of said semiconductor substrate would fall on a load element region associated therewith.
34. A semiconductor device according to claim 23, wherein each of the gate electrodes of said first and second MISFETs is disposed below at least a part of said second and third regions.
35. A semiconductor device according to claim 23, wherein said first wiring line is made integrally with said polysilicon film.
36. A semiconductor device according to claim 23, wherein said load element regions are load resistor means.
37. A semiconductor device according to claim 36, wherein said complementary data lines are formed from a first level layer of metal layer.
Description

This is a divisional of application Ser. No. 225,467, filed July 28, 1988, which is a divisional of application Ser. No. 899,404, filed Aug. 22, 1986, now U.S. Pat. No. 4,774,203.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor memory device and, more particularly, to a technique which may be effectively applied to a semiconductor integrated circuit device having a static random-access memory (hereinafter referred to as an "SRAM").

Each of the memory cells in an SRAM generally comprises a flip-flop and two transfer MISFET's. The flip-flop is composed of a pair of inverter circuits each comprising of a high-resistance load element and a drive MISFET, the input and output terminals of these inverter circuits being cross-coupled, and the transfer MISFET's are respectively connected a pair of input and output terminals of the flip-flop. The above-described high-resistance load element is constituted by a polysilicon film which is formed integral with wiring for the power supply voltage for the purpose of reducing the memory cell area.

The polysilicon film used as the high-resistance load element has no n-type impurity (i.e., As or P) introduced therein so that the film has a relatively high resistance. On the other hand, the polysilicon film used as the power supply voltage wiring has the above-described n-type impurity introduced therein. Such technique is disclosed in U.S. Pat. No. 4,554,729 (Japanese Patent Laid-Open No. 130461/1982).

SUMMARY OF THE INVENTION

For the purpose of reducing the area of a memory cell, a data line is formed such as to extend above the high-resistance load element through an insulator film. In such memory cell structure, a parasitic MISFET which has a channel region defined by the high-resistance load element is formed. This parasitic MISFET is constituted by a data line which serves as the gate electrode, and n-type polysilicon and single crystal silicon films which respectively serve as source and drain regions, those films being respectively connected to both end portions of the load element. The present inventors examined the parasitic MISFET and have found that a channel is formed in the high-resistance load element by the field effect produced by the operation of the data line, and this leads to an increase in the current flowing through the load element, resulting disadvantageously in an increase in the power dissipation.

In view of the above-described circumstances, it is a primary object of the present invention to provide a technique which may be effectively applied to an SRAM including of memory cells each having a high-resistance load element, the technique enabling prevention of increase in the current flowing through the above-described high-resistance load element, thereby reducing the power dissipation in the SRAM.

The above and other objects, novel features and advantages of the present invention will become clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.

A representative of the novel techniques disclosed in this application will be briefly summarized below.

In an SRAM including of memory cells each having a high-resistance load element, the load element is formed from a polysilicon film. An impurity which increases the threshold voltage of a parasitic MISFET having a channel region defined by the load element is introduced into at least a part of the polysilicon film. Alternatively, the deposition of the polysilicon film is carried out at a relatively high temperature. Thus, it is possible to prevent increase in the current flowing through the high-resistance load element and thereby to reduce the power dissipation in the SRAM.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a memory cell in an SRAM, which is employed to describe the present invention;

FIG. 2 is a plan view of a memory cell in an SRAM in accordance with a first embodiment of the present invention;

FIG. 3 is a sectional view taken along the line III--III in FIG. 2, which shows a memory cell in an SRAM in accordance with the first embodiment of the present invention;

FIGS. 4 to 6 are plan views of a part of the SRAM cell shown in FIG. 2;

FIG. 7 is a graph showing the relationship between the current flowing through the high-resistance load element and the voltage applied to the data line, which is employed to describe the present invention;

FIG. 8 is a graph showing the relationship between the current flowing through the high-resistance load element and the voltage applied to the data line, which is employed to describe a second embodiment of the present invention;

FIGS. 9 and 10 are a plan and a sectional view, respectively, of an essential part of a memory cell, which are employed to describe a further embodiment of the present; and

FIGS. 11 and 12 are a plan view and a sectional view, respectively, of an essential part of a memory cell, which are employed to describe a still further embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each of the memory cells which constitute an SRAM is provided at an intersection between a pair of complementary data lines DL, DL and a word line WL, as shown in FIG. 1.

The memory cell comprises a flip-flop and transfer MISFET's Qt1, Qt2 which are respectively connected between a pair of input and output terminals of the flip-flop and the data lines DL, DL. The word line WL is connected to the respective gates of the MISFET's Qt1 and Qt2. The flip-flop is composed of two inverter circuits, one of them comprising a high-resistance load element R1 and a drive MISFET Qd1, and the other comprising a high-resistance load element R2 and a drive MISFET Qd2. The input and output terminals of one inverter circuit are respectively coupled to the output and input terminals of the other inverter circuit. Each high-resistance load element R is connected at one end to a power supply voltage VCC and at the other end to the drain region of the corresponding drive MISFET Qd. The source region of each drive MISFET Qd is connected to a reference voltage VSS.

The power supply voltage VCC is, for example, the operating voltage, i.e., 5V, of the circuit, and the reference voltage VSS is, for example, the ground potential, i.e., 0V, of the circuit.

A plan view of an SRAM cell according to the present invention is shown in FIG. 2, and a cross-through through section of the cell line III--III in FIG. 2 is shown in FIG. 3. In addition, FIGS. 4 to 6 show in combination the patterns of conductor layers which constitute the memory cell and a field insulator film, for the purpose of facilitating understanding of the present invention. FIG. 4 is a plan view of the memory cell, in which the data lines DL and DL in the arrangement shown in FIG. 2 are omitted. FIG. 5 is a plan view of the memory cell, in which the second-level polysilicon layer and two mask patterns for impurity introduction in the arrangement shown in FIG. 4 are omitted, the second-level polysilicon layer and the mask patterns being shown in FIG. 6. It should be noted that any insulator film provided between each pair of adjacent conductor layers other than the field insulator film is not shown in FIGS. 2, 4 and 5 for the purpose of facilitating the understanding of the arrangement of this embodiment.

In FIGS. 2 to 5, the reference numeral 1 denotes an n- -type semiconductor substrate made of single crystal silicon, and the numeral 2 denotes a p- -type well region. The memory cell is formed within the well region 2. Circuits other than the memory cell are complementary MISFET circuits. The N-channel MISFET is formed within a p- -type well region which is different from the p- -type well region 2 in which is provided the memory cell, while the P-channel MISFET is formed in either the n- -type semiconductor substrate 1 or an n- -type well region having an impurity concentration which is different from that of the substrate 1. The reference numeral 3 denotes a field insulator film, and the numeral 4 denotes a p-type channel stopper region. Both the field insulator film 3 and the channel stopper region 4 are formed on the main surface of the semiconductor substrate 1 so as to isolate semiconductor elements such as MISFET's from each other. The configuration of the field insulator film 3 in the memory cell will be clearly understood with reference to FIG. 5.

The MISFET's Qd1, Qd2, Qt1 and Qt2 which constitute the memory cell are formed within the well region 2 and each constituted by a gate insulator film, 5, a gate electrode 6A, and a pair of n- - and n+ -type semicondcutor regions 7 and 9 which respectively serve as source and drain regions, or vice versa.

The gate electrode 6A is constituted by a composite film (i.e., a polycide film) constructed of a polysilicon (polycrystalline silicon) film and a silicide film of a refractory metal (Mo, Ta, Ti or W) provided thereon. The polysilicon film has an impurity (As or P) introduced therein for the purpose of reducing the resistance value thereof. The gate electrode 6A may be constituted by any one of the following films, that is, a single layer of a refractory metal film or a refractory metal silicide film, or a composite film consisting of a polysilicon film and a refractory metal film provided thereon. The respective gate electrodes 6A of the MISFET's Qt1 and Qt2 are formed integral with each other and extended in the horizontal direction as viewed in the drawings to form a word line WL. The respective gate electrodes 6A of the MISFET's Qd1 and Qd2 are directly connected to the semiconductor regions 9 (and 7) of the other MISFET's through contact holes 5A formed in the gate insulator film 5. Thus, the cross-coupling of the flip-flop in the memory cell is realized. A reference voltage wiring 6B is formed in the same layer as the gate electrode 6A and made of the same conductive material as that of the gate electrode 6A, the wiring 6B extending parallel to the word line WL. The reference voltage wiring 6B is directly connected to the semiconductor regions 9 which constitute the respective sources of the MISFET's Qd1 and Qd2 through contact holes 5A provided in the gate insulator film 5.

Each semiconductor region 7 is provided between the channel region and the semiconductor region 9 of each of the MISFET's Qt and Qd so as to constitute a part of either the source or drain region thereof. The semiconductor regions 7 are formed by ion implantation using the gate electrodes 6A as a mask, the regions 7 being self-aligned with respect to the gate electrodes 6A. The semiconductor regions 9 are formed by ion implantation using as a mask the gate electrodes 6A and insulator films (side wall spacers) 8 formed on the side walls of the gate electrodes 6A, the regions 9 being self-aligned with respect to these portions. The insulator film 8 is made of, for example, a silicon oxide film, which can be formed using reactive ion etching. The semiconductor regions 7 and 9 constitute in combination a MISFET of the LDD (Lightly Doped Drain). A p+ -semiconductor region 20 is formed underneath each of the regions 7 and 9 which are storage nodes of the memory cells, thereby increasing the junction capacitance, and thus preventing the occurrence of any soft error which might otherwise be caused by α rays.

FIG. 5 clearly shows the respective patterns of the contact holes 5A, the first-level conductor layers 6A, 6B and the semiconductor regions 9 of the memory cell, that is, the respective configurations and connections of the MISFET's Qd1, Qd2, Qt1 and Qt2. The insulator films 8 and the semiconductor regions 7 are not shown in FIG. 5 (and FIGS. 2 and 4).

The reference numeral 10 denotes an insulator film which is made of, for example, a silicon oxide film, the insulator film 10 being provided above each of the semiconductor region 9 and above the electrodes or wirings 6A, 6B. The reference numeral 11 denotes an insulator film which is made of, for example, a phosphosilicate glass (PSG) film, and which is provided over the whole surface of the semiconductor substrate 1 in such a manner as to cover the MISFET's Qt and Qd. Contact holes 12 are provided by removing the insulator films 10 and 11 above predetermined semiconductor regions 9.

On the insulator film 11 are formed high-resistance load elements R1 and R2 and a power supply voltage wiring, which are composed of the same level layer made of the same material as each other. More specifically, they are formed from a polysilicon layer which is a second-level conductor layer 13 of the memory cell. One end of the load element R1 is connected to the semiconductor region 9 of the MISFET Qt1 and the gate electrode 6A of the MISFET Qd2 through the corresponding contact hole 12, while one end of the load element R2 is connected to the semiconductor region 9 of the MISFET Qt2 and the gate electrode 6A of the MISFET Qd1, and the other ends of the load elements R1 and R2 are connected to the power supply voltage wiring which is formed integral with the load elements R1 and R2.

As shown in FIGS. 2 to 6, each of the high-resistance load elements R1 and R2 comprises a polysilicon film 13A which has no impurity introduced therein for the purpose of reducing the resistance value thereof, that is, an intrinsic (i) polysilicon film 13A, and a (p) polysilicon film 13B which has a p-type impurity introduced therein. The polysilicon film 13A is provided on the power supply voltage wiring 13C side of each load element R, and the polysilicon film 13B is provided on that side of the load element R which is connected to the semiconductor region 9 of the corresponding transfer MISFET Qt. The power supply voltage wiring 13C is formed by an n+ -type polysilicon layer which has an n-type impurity, e.g., arsenic or phosphorus, heavily doped therein for the purpose of minimizing the resistance value thereof.

The polysilicon film 13A is formed within a region surrounded by the dotted line denoted by the reference numeral 17 in FIGS. 2, 4 and 6. The polysilicon film 13B is formed within a region surrounded by the dotted line denoted by the reference numeral 18 in FIGS. 2, 4 and 6. In other words, the load elements R1 and R2 are formed in such a manner that the p-type polysilicon film 13B is provided in a part of the i-type polysilicon film 13A (i.e., the i-p structure).

Introduction of an impurity for forming the region 13B of the polysilicon film 13 is carried out in a state wherein the region other than the region surrounded by the dotted line 18 is covered with a mask. The polysilicon film 13B is formed by, for example, introducing a p-type impurity, e.g., boron fluoride (BF2), into the polysilicon film 13 at 60 KeV and 21012 atoms/cm2 using the ion implantation technique. Thereafter, introduction of an impurity for forming the region 13C in the polysilicon film 13 is carried out in a state wherein the region surrounded by the dotted line 17 is covered with a mask.

The power supply voltage wiring 13C is formed by, for example, introducing an n-type impurity, e.g., arsenic (As) into the polysilicon film 13 at 50 KeV and 51015 atoms/cm2 using the ion implantation technique. Phosphorus (P) may also be employed as an n-type impurity. As the two masks 17 and 18, for example, a resist mask may be employed.

The configuration of the polysilicon film 13 and the discrimination between the regions 13A, 13B and 13C will be clearly understood from FIG. 6.

The reference numeral 14 denotes an insulator film which is made of a silicon oxide film formed over the whole surface of the semiconductor substrate 1 in such a manner as to cover the load elements R1 and R2. The numeral 15 denotes contact holes which are provided by removing the insulator films 10, 11 and 14 above the semiconductor region 9 of each MISFET Qt. The numeral 16 denotes data lines DL and DL which are respectively connected to the semiconductor regions 9 of the MISFET's Qt through the contact holes 15, the data lines 16 extending over the insulator film 14. The data lines 16 are formed from an aluminum film or an aluminum film containing predetermined additives (Si and/or Cu).

The arrangement wherein at least a part of each of the load elements R1 and R2 is constituted by the polysilicon film 13B having a p-type impurity introduced therein enables an increase in the threshold voltage of a parasitic MISFET which is formed using the load element R1 or R2 as its channel region, as shown in FIG. 7. The parasitic MISFET may be regarded as formed using the insulator film 14 as the gate insulator film, the data line 16 as the gate electrode, the power supply voltage wiring 13C as the drain region, and the semiconductor region 9 of the MISFET Qt as the source region. The threshold voltage of the prasitic MISFET can be set such as to be higher than the voltage (e.g., 0 to 5V) applied to the data line 16 which serves as the gate electrode of the parasitic MISFET. Accordingly, it is possible to prevent any increase in the current flowing through the load elements R1 and R2 due to the field effect, so that the power dissipation in the SRAM can be reduced.

FIG. 7 is a graph showing the relationship between the current flowing through each of the load element R1 and R2 and the voltage applied to the corresponding data lines 16. The abscissa axis represents the gate voltage of the parasitic MISFET, that is, the voltage applied to the data line 16, and the coordinate axis represents the current flowing through the parasitic MISFET, that is, the current (A) flowing through the load element R1 (R2). The curve I shows data measured when the whole area of the load element R1 (R2) is constituted by an i-type polysilicon film 13A which has no impurity introduced therein. The curve II shows data measured when the whole area of the load element R1 (R2) is constituted by the p-type polysilicon film 13B having a p-type impurity introduced therein. The curve III shows data measured with respect to the SRAM according to the present invention in which the load element R1 (R2) is constituted by the i- and p-type polysilicon films 13A and 13B.

As will be clear from FIG. 7, by forming at least a part of the load element R1 (R2) from the p-type polysilicon film 13B, it is possible to set the threshold voltage of the parasitic MISFET so as to be higher than the voltage applied to the data line 16, as described above.

In addition, it is possible to increase the breakdown voltage at the junction (n+ -i junction) between the power supply voltage wiring 13C and the polysilicon film 13A, together with said threshold voltage, by arranging the load element R1 (R2) such that the i-type polysilicon film 13A is provided on that side of the load element which is connected to the n+ -type power supply voltage wiring 13C and the p-type polysilicon film 13B is provided on that side of the load element which is connected to the semiconductor region 9. The whole area of the load element R1 (R2) may be constituted by the p-type polysilicon film 13B. However, it is preferable to arrange the load element R1 (R2) in such a manner that the i-type polysilicon film 13A is provided on that side of the load element which dominates the breakdown voltage, and the p-type polysilicon film 13B on that side of the load element which dominates the threshold voltage. In other words, it is possible to optimize the concentration of a p-type impurity which is introduced into the polysilicon film 13B without taking the breakdown voltage into consideration.

According to a second embodiment of the present invention, the threshold voltage of the parasitic MISFET whose channel region is defined by a high-resistance load element which constitutes a part of an SRAM cell is increased by optimizing the conditions in which the load element is formed.

FIG. 8 shows the relationship between the current flowing through the load element and the voltage applied to the data line for explaining the second embodiment.

The curve IV shows data measured when the load elements R1 and R2 are constituted by an i-type polysilicon film 13A which is deposited by the CVD technique at a temperature of 570 C. and which has no impurity introduced therein for the purpose of lowering the resistance value thereof. This polysilicon film 13A has a thickness of about 1000 Å. The polysilicon film 13A, which is deposited at the above-described temperature, is amorphous.

The curve V, VI and VII shows data measured when the load elements R1 and R2 are constituted by an i-type polysilicon film 13A which is deposited by the CVD technique at a relatively high temperature, about 640 C., and which has no impurity introduced therein for the purpose of lowering the resistance value thereof. The polysilicon film 13A shown by the curve V has a thickness of about 2000 Å, while the polysilicon film 13A shown by the curve VI has a thickness of about 1000 Å, and the polysilicon film 13A shown by the curve VII has a thickness of about 500 Å. The polysilicon film 13A, which is deposited at the above-described temperature, is polycrystalline.

As will be clear from the curves VI and VII in FIG. 8, when the load elements R1 and R2 are constituted by a polysilicon film 13A which is formed at a relatively high temperature, about 640 C., and with a thickness of about 1000 Å or less, the threshold voltage of the load elements R1 and R2 can be made higher than that of those which are constituted by a polysilicon film 13A formed at a temperature of about 570 C. This threshold voltage is higher than the voltage (e.g., 5V) which is applied to the data lines 16. Accordingly, it is possible to prevent any increase in the current flowing through the parasitic MISFET's whose channel regions are respectively defined by the load elements R1 and R2, and this enables a reduction in the power dissipation in the SRAM.

The load elements R1 and R2 are formed at a temperature of about 600 C. or higher. The orientation and deposition state of the silicon crystal change at about 570 to 600 C. Accordingly, the load elements R1 and R2 are formed at a relatively high temperature i.e., 600 C. or higher, in order to make the polysilicon film 13A polycrystalline and increase the threshold voltage. It is preferable to form the load elements R1 and R2 at a temperature of about 700 C. or lower. This is because a temperature higher than 700 C. increases the deposition rate of the polysilicon film 13A, and this leads to various problems such as nonuniformity in the film thickness.

It should be noted that, when the polysilicon film is positively utilized as the channel region of a MISFET in place of the parasitic MISFET, the polysilicon film is formed at a temperature of 570 C. or lower. This is because such relatively low temperature enables lowering in the state of the polysilicon film and it is therefore possible to lower the threshold voltage and thereby obtain excellent MIS characteristics.

According to the second embodiment, it is possible to increase the threshold voltage of the parasitic MISFET's whose channel regions are respectively defined by the load elements R1 and R2.

The above-described SRAM is basically mentioned in U.S. patent application Ser. No. 764,208 filed on Aug. 8, 1985 cited in this specification as a reference in place of the description in this specification.

According to the first embodiment, the arrangement of the load elements R1 and R2 is changed as described above in accordance with the present invention. In consequence, the method of forming the load elements R1 and R2 is changed. More specifically, the polysilicon film 13 is formed over the whole surface of the semiconductor substrate 1 by CVD and patterned in the shape shown in FIG. 6, and the first resist mask 18 is then formed. After the ion implantation of a p-type impurity using the mask 18, the mask 18 is removed, and the second resist mask 17 is newly formed. Then, ion implantation of an n-type impurity is carried out using the mask 17.

According to the second embodiment, CVD for forming the polysilicon film 13 employed to constitute the load elements R1 and R2 is carried out at the above-described temperature, and the thickness of the film 13 is also limited.

By virtue of the novel technique disclosed in this application, the following advantages are obtained.

(1) A high-resistance load element which constitutes a part of an SRAM cell is formed from a polysilicon film, and an impurity of a predetermined conductivity type is introduced into a part of the polysilicon film, whereby it is possible to increase the threshold voltage of a parasitic MISFET which is formed using the load element as its channel region.

(2) The load element is formed from a polysilicon film which is formed at a relatively high temperature, about 600 C. or higher, and with a thickness of about 1000 Å or less, whereby it is possible to increase the threshold voltage of a prasitic MISFET formed using the load element as its channel region.

(3) Since it is possible to prevent any increase in the current flowing through the load element by virtue of the advantages (1) and (2), the power dissipation in the SRAM can be reduced.

Although the invention accomplished by the present inventors has be described above in detail by way of the embodiments, it is a matter of course that the present invention is not necessarily limitative to the above-described embodiments and various changes and modifications may be imparted thereto without departing from the gist of the invention.

The position at which the p-type impurity-doped region 13B is formed may be variously changed. Examples thereof are shown in FIGS. 9 to 12. FIGS. 9 and 11 correspond to FIG. 6, and FIGS. 10 and 12 correspond to FIG. 3.

FIGS. 9 and 10 show in combination an example in which the p-type polysilicon film 13B is formed in such a manner that it is not in contact with the n+ -type semiconductor region 9. More specifically, the polysilicon film 13 has a structure in which the conductivity type thereof changes from the power supply voltage wiring side in the following sequence: n+ -type - i-type - p-type - n+ -type. In consequence, the patterns of the masks 17 and 18 are changed as shown in FIG. 9. As a result, the cross-sectional configuration of the polysilicon film 13 is such as that shown in FIG. 10. More specifically, the semiconductor region 9 and the n+ -type polysilicon film 13C are in contact with each other. Thus, it is possible to prevent the p-type impurity in the polysilicon film 13B from being diffused into the n-type semiconductor region. In addition, since no pn junction is formed at the boundary between the polysilicon film 13 and the semiconductor substrate 1, it is possible to obtain a stable pn junction involving less leakage current.

FIGS. 11 and 12 show in combination an example in which the p- and n+ -type polysilicon films 13B and 13C are formed using only one mask as shown in FIG. 11. As will be clear from FIG. 11, the polysilicon film 13 has a structure in which the conductivity type thereof changes from the power supply voltage wiring side in the following sequence: n+ -type - p-type - i-type - p-type - n+ -type. Since the regions 13B and 13C are formed using a single mask, the manufacturing process is changed as follows. After the patterning of the polysilicon film 13, a mask 19 made of, for example, a resist film is formed as shown in FIG. 11. In this state, boron fluoride ions and arsenic ions are implanted under the above-described conditions, respectively, and then, a heat treatment is carried out. In consequence, the boron fluoride and arsenic are horizontally diffused in the polysilicon film 13. There is a difference between the diffusion rate of boron and that of arsenic, that is, the former is larger than the latter. Therefore, the p-type region 13B is formed by making use of this fact. As a result, the cross-sectional configuration of the polysilicon film 13 is such as that shown in FIG. 12. This method enables a reduction in the number of required manufacturing steps. In addition, since it becomes unnecessary to provide the alignment margin between the masks 17 and 18, that is, since the regions 13A, 13B and 13C are self-aligned, this method is advantageous in realization of a very fine or minute device.

The p-type region 13B may be formed in the center of the i-type region 13A. More specifically, the polysilicon film 13 may have a structure in which the conductivity type thereof changes from the power supply voltage wiring side in the following sequence: n+ -type - i-type - p-type - i-type - n+ -type. Such structure can be realized by appropriately modifying the patterns of the masks 17 and 18.

The first embodiment or the above-described modifications of the first embodiment may be combined with the second embodiment. More specifically, the load elements R1 and R2 may be formed in the manner shown in the first embodiment or the modifications thereof using a plysilicon film which is deposited at 600 to 700 C. and with a thickness of about 1000 Å or less.

The present invention may be widely applied not only to memory IC's having memory cells each including polysilicon with a high resistance, but also to, for example, logic IC's in which the above-described memory cell is formed in a part on the same substrate.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US4322821 *Dec 19, 1979Mar 30, 1982U.S. Philips CorporationMemory cell for a static memory and static memory comprising such a cell
US4480319 *Jan 26, 1981Oct 30, 1984Hitachi, Ltd.Emitter coupled flip flop memory with complementary bipolar loads
US4538244 *Dec 27, 1984Aug 27, 1985Fujitsu LimitedSemiconductor memory device
US4774203 *Aug 22, 1986Sep 27, 1988Hitachi, Ltd.Method for making static random-access memory device
US4814841 *Jan 30, 1987Mar 21, 1989Kabushiki Kaisha ToshibaSemiconductor device
US4828629 *Mar 20, 1986May 9, 1989Hitachi, Ltd.Process of fabricating silicon oxide and gettering films on polycrystalline silicon resistance element
US4841481 *Jul 28, 1988Jun 20, 1989Hitachi, Ltd.Semiconductor memory device
JPH0289157A * Title not available
Non-Patent Citations
Reference
1 *Semiconductor Devices Physics And Technology, S. M. Sze.
Referenced by
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US5744846 *Jan 3, 1997Apr 28, 1998Micron Technology, Inc.SRAM cell employing substantially vertically elongated pull-up resistors and methods of making, and resistor constructions and methods of making
US5751050 *Oct 24, 1996May 12, 1998Nec CorporationSemiconductor device having a polysilicon resistor element with increased stability and method of fabricating same
US5757053 *Apr 19, 1996May 26, 1998Taiwan Semiconductor Manufacturing Company Ltd.Effective load length increase by topography
US5847412 *Jun 3, 1997Dec 8, 1998Kabushiki Kaisha ToshibaSemiconductor device and a method for manufacturing the same
US5847437 *Dec 23, 1996Dec 8, 1998National Science CouncilRare-earth element-doped III-V compound semiconductor schottky diodes and device formed thereby
US5939760 *Mar 26, 1998Aug 17, 1999Micron Technology, Inc.SRAM cell employing substantially vertically elongated pull-up resistors and methods of making, and resistor constructions and methods of making
US5981329 *Jul 29, 1997Nov 9, 1999Micron Technology, Inc.SRAM cell employing substantially vertically elongated pull-up resistors and methods of making, and resistor constructions and methods of making
US6013934 *Mar 18, 1997Jan 11, 2000Lucent Technologies Inc.Semiconductor structure for thermal shutdown protection
US6043117 *Jul 27, 1999Mar 28, 2000Micron Technology, Inc.SRAM cell employing substantially vertically elongated pull-up resistors and methods of making, and resistor constructions and method of making
US6242781Jul 22, 1997Jun 5, 2001Micron Technology, Inc.Resistor constructions
WO2001019258A1Sep 14, 2000Mar 22, 2001Abbott LaboratoriesKnot tying device and method
Classifications
U.S. Classification257/380, 257/E21.004, 257/E21.661, 257/904, 257/369, 257/E27.101
International ClassificationH01L27/11, H01L21/8244, H01L21/02
Cooperative ClassificationY10S257/904, H01L27/1112, H01L27/11, H01L28/20
European ClassificationH01L28/20, H01L27/11
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